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H. Uozato, T. Kawamorita; Evaluation of Corneal Shape and Power With Pentacam . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1175.
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To investigate the shape and the astigmatism of the cornea in the estimated K value regarded as a single refractive surface with the effective refractive index 1.3375 and the True Net power calculated in consideration of posterior corneal surface and corneal thickness.
Fifty six eyes from 56 subjects (mean age 23.2±1.5) with no known abnormalities were included in the study. None of them were rigid contact lens wearers; soft contact lens wearers were included but removed their contact lenses at least 24 hours before the study. Measurements of corneal shape were performed with Pentacam device (Oculus, Germany), which is comprised of a rotating Scheimpflug camera and short–wavelength slit light. As the refractive index of cornea, aqueous humor and air, we used 1.376, 1.336, and 1.000, respectively.
The average estimated K value for the central cornea 3.0 mm (42.92±1.59 D) were significantly larger than the average True Net power (41.71±1.59 D) (p<0.01, Wilcoxon Signed Rank Test). The average estimated astigmatism (1.38±0.73 D) were significantly larger than the astigmatism calculated with True Net power (1.02±0.70 D) (p<0.01, Wilcoxon Signed Rank Test). The astigmatism of the posterior cornea compensated the astigmatism of the anterior corneal surface with approximately 42 % on an average.
These results suggest that the corneal refractive power estimated with the effective refractive index is overestimated approximately 1.2 D in the central corneal power and 0.4 D in the astigmatism.
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